1. Field of the Invention
The present invention relates generally to aluminum alloys and more particularly, to such alloys, their methods of manufacture and use, particularly in the aerospace industry.
2. Description of Related Art
Continuous efforts are being directed towards the development of materials that could simultaneously reduce weight and increase structural efficiency of high-performance aircraft structures. Aluminum-lithium (AlLi) alloys are very appealing regarding this target because lithium can reduce the density of aluminum by 3 percent and increase the elastic modulus by 6 percent for every weight percent of lithium added. However, AlLi alloys have yet to be extensively used in the aircraft industry due to several drawbacks of early generation alloys such as, for example, inadequate thermal stability, anisotropy and inadequate fracture toughness.
The history of AlLi alloys development is discussed, for example, in a chapter “Aluminum-Lithium Alloys”: of the book Aluminum and Aluminum Alloys, (ASM Specialty Handbook, 1994). The first aluminum-lithium alloys (Al—Zn—Cu—Li) were introduced German inventors in the 1920s, followed by the introduction of alloy AA2020 (Al—Cu—Li—Mn—Cd) in the late 1950s and the introduction of alloy 1420 (Al—Mg—Li) in the Soviet Union in the mid-1960s. The only industrial applications of alloy AA2020 were the wings and horizontal stabilizers for RA5C Vigilante aircraft. A typical composition for alloy AA2020 was (in weight percent) Cu: 4.5, Li: 1.2, Mn: 0.5, Cd: 0.2. There were various reasons for the limited applications of the AA2020 alloy, for example, the fact AA2020 exhibited shortcomings in fracture toughness. In addition to the specific effect of Cd, the use of Mn in this alloy was assessed to be one of the reasons of its limited properties. In 1982, E. A. Starke stated (in Metallurgical Transactions A, Vol 13A, p 2267) “The larger Mn-rich dispersoids may also be detrimental to ductility by initiating voids”. This idea of a detrimental effect of Mn was broadly recognized by those skilled in the art. For example, in 1991, Blackenship stated (in Proceedings of the Sixth International Aluminum-Lithium Conference, Garmisch-Partenkirchen, p 190), “Manganese-rich dispersoids nucleate voids and thus encourage the fracture process”. It was suggested that zirconium should be used instead of manganese for grain structure control. In the same document, Blackenship stated, “zirconium is the alloying element of choice for grain structure control in Al—Li—X”.
The development of AlLi alloys continued in the 1980s and led to the introduction of commercial alloys AA8090, AA2090 and AA2091. All these alloys contained zirconium instead of manganese.
In the early 1990s, a new family of AlLi alloys containing silver known under the trademark “Weldalite”® was introduced. These alloys typically contained lower Li and exhibited better thermal stability. U.S. Pat. No. 5,032,359 (Pickens, Martin Marietta) describes alloys containing from 2.0 to 9.8 weight percent of an alloying element consisting of Cu, Mg and mixtures thereof, from 0.01 to 2.0 weight percent of Ag, from 0.2 to 4.1 weight percent of Li and from 0.05 to 1.0 weight percent of a grain refiner additive selected from Zr, Cr, Mn, Ti, B, Hf, V, TiB2 and mixtures thereof. It should be noted that the list of grain refiners proposed by Pickens actually mixes elements used for foundry grain refining (such as TiB2) and elements used for grain structure control during the transformation operations such as zirconium. Even though Pickens stated that, “although emphasis herein shall be placed upon use of zirconium for grain refinement, conventional grain refiners such as Cr, Mn, Ti, B, Hf, V, TiB2 and mixtures thereof may be used”, it clearly appears from the history of AlLi alloy development that a prejudice against the use of any element other than Zr for grain structure control existed to the one skilled in the art. Indeed, in all of the examples described by Pickens, Zr was used.
Use of zirconium for grain refining can also be found in an alloy developed more recently (AA2050, see also WO2004/106570), manganese addition being used to improve toughness. In AA2297, which contains lithium, copper, manganese and optionally magnesium but no silver, zirconium is also used for grain refining. U.S. Pat. No. 5,234,662 discloses a preferred composition of 1.6 wt. % Li, 3 wt. % Cu, 0.3 wt. % Mn and 0.12 wt. % Zr. AA2050 and AA2297 alloys have been mainly proposed for thick plates, with a gauge higher than 0.5 inch.
Another family of AlLi alloys, which contained Zn, was described for example in U.S. Pat. No. 4,961,792 and U.S. Pat. No. 5,066,342 and developed in the early 1990s. The metallurgy of these alloys cannot be compared to the metallurgy of “Weldalite”® alloys because the incorporation of a significant amount of zinc, and in particular the combination of zinc with magnesium, significantly modifies the properties of the alloy, for example in terms of strength and corrosion resistance.
In order to use AlLi alloys for fuselage skin applications, the alloys should reach the same or even better performances in strength, damage tolerance and corrosion resistance than currently used Li-free alloys. In particular, resistance to fatigue crack growth is a major concern for those applications and that explains why alloys recognized for their high damage tolerance, such as AA2524 and AA2056 alloys, are traditionally used. Weldability and corrosion resistance are also among other desirable properties. With the increasing trend to reduce costly mechanical fastening operations in the aircraft industry, weldable alloys such as AA6013, AA6056 or AA6156 are introduced for fuselage skin panels. High corrosion resistance is also desirable in order to substitute clad products with less expensive bare products.
It was known that Al—Li alloys often have problems in terms of anisotropy in tensile properties, which in turn, governs the extent of anisotropy in the other mechanical properties. Low yield strength at intermediate test directions, for example 45° to the rolling direction, is a prominent manifestation of the anisotropy.
As far as damage tolerance properties are concerned, the development of an R-Curve is a widely recognized method to characterize fracture toughness properties. The R-curve represents the evolution of the effective stress intensity factor for crack growth as a function of effective crack extension, under increasing monotonic loading. The R-curve enables one to determine the critical load for unstable fracture for any configuration relevant to cracked aircraft structures. The values of stress intensity factor and crack extension are effective values as defined in the ASTM E561 standard. The generally employed analysis of conventional tests on center cracked panels gives an apparent stress intensity factor at fracture [Kapp]. This value does not necessarily vary significantly as a function of R-curve length. However the length of the R-curve—i.e. maximum crack extension of the curve—is an important parameter in itself for fuselage design, in particular for panels with attached stiffeners.
There is a need for a high strength without anisotropy, high fracture toughness, and especially high crack extension before unstable fracture, high corrosion resistance, low density (i.e. not more than about 2.70 g/cm3) Al—Cu—Li alloy for aircraft applications, and in particular for fuselage sheet applications.